1. Field of the Invention
The present invention relates to a waveguide which guides light to an intended position, and to a thermally-assisted magnetic recording head which writes data by leading a light for thermal-assist to the opposed-to-medium surface side of the head by means of the waveguide.
2. Description of the Related Art
As the recording density of a magnetic recording apparatus becomes higher, as represented by a magnetic disk apparatus, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. Especially, in the magnetic recording medium, it is necessary to decrease the size of magnetic microparticles that constitutes the magnetic recording layer of the medium, and to reduce irregularity in the boundary of record bit in order to improve the recording density. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium, where data is to be written; just after that, writing is performed by applying write field to the heated portion.
The heating of the portion to be written of the medium is performed by irradiating the portion with near-field light or with laser light. In the case of heating with near-field light, as described in, for example, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894, a near-field light generator as a conductive plate, so-called a plasmon antenna, is provided on the opposed-to-medium surface, then near-field light is generated by irradiating the opposite side to the opposed-to-medium surface of the plasmon antenna with laser light guided through a waveguide. In the case of heating with laser light, the magnetic recording medium is directly irradiated with laser light guided through a waveguide. In this way, a waveguide is an important component in any case.
In the thermally-assisted magnetic recording head, the waveguide and a main magnetic pole of the write head element for generating write magnetic field (write field) are disposed close to each other. For example, the thermally-assisted magnetic recording that uses the plasmon antenna applies thermal-dominant technique in which spatial resolution of record bits depends on the spot diameter of near-field light. Therefore, temperature gradient adjacent to the irradiating center of near-field light becomes significantly large. While, magnetic-field gradient of write field generated from the main magnetic pole is set to be considerably large according to the higher recording density. As a result, in writing record bits, the irradiating center of near-field light, or the plasmon antenna, is required to be sufficiently close to the main magnetic pole in order to obtain a write field with sufficient intensity near the irradiating center. Accordingly, the position of the emitting spot center of light propagating through the waveguide must be set to be sufficiently close to the main magnetic pole.
Whereas, also in the case of directly irradiating the magnetic recording medium with laser light guided through a waveguide, a sufficient intensity of write field must be applied to the laser light spot center and its neighborhood in order to write record bits. For this purpose, the position of the emitting spot center of light propagating through the waveguide must be set to be sufficiently close to the main magnetic pole.
The methods for setting the position of the emitting spot center of light propagating through the waveguide to be sufficiently close to the main magnetic pole include: 1) reducing the thickness in the stacking direction of the waveguide; and 2) using a waveguide with multilayered structure. The method 2) intends to control the position of the light spot center on the light-emitting end of the waveguide by appropriately designing the refractive index of each layer in the multilayered structure. However, in the case of the method 1), the reduction of the total thickness causes the thickness of the light-receiving end where the waveguide receives laser light from a light source to become smaller. As a result, optical coupling efficiency between the light source such as a laser diode and the waveguide is degraded, that is, the optical coupling loss therebetween is increased; thus it may become difficult to introduce a sufficient intensity of light into the waveguide. Further, in the case of the method 2), the position of the light spot center on the light-emitting end deviates significantly from the intended position due to a slight displacement of the light source in relation to the light-receiving end of the waveguide. Therefore, there may be a difficult problem that the position accuracy of the light source in relation to the light-receiving end must be sufficiently high.